tivity of modern agriculture that we observe occurring ever more widely throughout the world. It is a dynamic process, and a major source of the economic dynamics is the advance in scientific and technical knowledge that becomes embodied in new agricultural inputs. As modernization proceeds, the economic importance of land—measured by what it contributes to total agricultural production-declines in relation to the importance of other inputs. High-yielding crops, chemical fertilizers, pesticides, irrigation, and the farming skills to manage them all act as "substitutes" for land. As a consequence, the limitational attribute of land has been modified, making it less severe, markedly so in the technically advanced countries (9, 10). Here too, there is need for caution in interpreting the age-old stranglehold that land has held on the supply of food.* ... Yet no less an economist than Colin Clark , no longer ago than 1941, in his book, The Economics of 1960, came to the conclusion that the world was in for a dramatic rise in the relative price of primary products. Clark did not come to this conclusion by indulging in some easy, intuitive guesses, nor did he rely on a simple projection of past trends. He drew upon his vast stock of data; he proceeded to put them into his "analytical model" with its strong bent for diminishing returns against land, and ground out the following conclusion for 1960. ". . .the terms of trade for primary produce will improve by as much as 90 percent from the average of 1925-34." To speak of so violent a rise in the relative price of primary products as an "improvement" is a neat twist. . . . But what are the facts as of 1960? Clark missed the price target altogether; his shot went off into space in the wrong direction. What went wrong? Did he assume too large a rise in population? On the contrary, the up-surge in population has been much greater than he assumed it would be. Has there been much less industrialization than he anticipated? Again, the answer is in the negative. Clark simply assumed a lot of secular diminishing returns against land and this assumption turned out to be invalid. (9, pp. 25-26; see also 12.) Long-Range vs. Short-Range Projections Long-range projections of the supply and demand for food are subject to all manner of doubts. Nevertheless, the propensity of economists to avoid long-range projections is viewed by some biologists and some demographers as one of the serious weaknesses of economics in facing up to the increasing pressure of population on primary (natural) resources. The sins attributed to economics are that it is shortsighted, all too practical, and overcommitted to monetary considerations, thus not sufficiently in touch with real biological *The standard assumption of the highly inelastic nature of the supply of farmland and the implied rise in the supply price of the products from land as industrialization proceeds and populations increase persists.